Improving dirt pickup resistance especially in waterborne coatings is critical to future coating innova-tions. This article focuses on resins used in the coatings with the aim of enhancing dirt pick up resistance.It presents initially optimization of an acrylic (BA-St-AA) copolymer synthesized through free-radicalemulsion polymerization and also, a polydimethylsiloxane (PDMS) homopolymer via cationic ring open-ing macro-emulsion polymerization of octamethyl cyclotetrasiloxane (D4). Concentration of monomers,Type of anionic emulsifiers, impeller stirrer speed and reaction temperature were considered as vari-ables in order to reach a high yield of conversion and colloidal stability. The polymers were characterizedusing fourier transfer infrared spectroscopy, gel permeation chromatography and particle size analysis.The optimized latexes were then blended in different ratios (0/100–100/0) and the properties of the filmswere assessed using optical microscopy, Si-mapping EDX analysis, contact angle measurement, water up-take, and color measuremets as well as stress strain analysis. The characterization results revealed latexeswith optimized conversion and uni-modal particle size distribution (PSD) were synthesized (PDMS: PSD25–250 nm, Mn 129000, PDI 2.23 and Acrylic: PSD 75–400 nm, Mn 61,000, and PDI 4.95). It was found
that 10–28 wt% PDMS in blends provides continuous and clear films with low dirt pickup properties. Theoptimized latex blend film showed a drying time of 25 min, contact angle of 96◦, water uptake of 27% after6 days, tensile strength of 7 MPa, and elongation at break of 690%. The results confirmed that blendingacrylic and silicone latexes in bi-modal PSD and optimized ratios leads to a good performance exteriorand uniform hydrophobic coating.
Recent developments in exterior coatings have heightenedhe need for superior weather and dirt resistant coatings [1–3].mproved dirt pick-up resistant coatings are achieved by differ-nt methods. One method is to prepare paint formulations withigment volume concentration (PVC) at below critical PVC (CPVC)4,5]. As PVC increases above CPVC, void space and porosity in theoating increase while overall performance including dirt pick-upesistance decreases. Another approach is to use a self chalkingoating in which the coatings film being eroded in exposure totmospheric oxygen, UV radiation, heat and rainfall. Erosion leads
o a new surface in the coatings with a clean appearance. This
ethod suffers from relatively slow erosion rate as well as theeakness of coating’s gloss retention. A further approach is treat-
∗ Corresponding author.E-mail address: [email protected] (S. Pazokifard).
ing the surface of the coatings so that the adhesion of soil/dirt/dustparticles is weakened [6]. This method is limited to requiring anadditional coating to be applied. Increase in Tg, cross-link densityand decrease in surface energy of the coating and also, blendingharder polymers or additives into the coating composition are allvarious methods which has been used to reduce dirt or soil accu-mulation on the coatings. Obviously, dirt pickup resistance of a filmcan be affected by surface roughness of the coating.
In latex paints, different factors affect dirt pickup properties,for instance, Tg, morphology and hardness of the resins used incoatings, as well as film formation properties of the coatings [7–9].Waterborne coatings have generally more potential to adsorb dirtin comparison with the solvent based ones, because of dirt adsorp-tion of the capillaries, formed during the film formation of latexpaints [10,11].
Waterborne acrylic coatings are ideal for their inertness, benignto environment and excellent color retention when exposed to out-door conditions. Styrene acrylic copolymers have been extensivelyused for this purpose due to their specific features [12–14]. How-
ver, dirt pick-up and mechanical properties are still a pressingroblem in these coatings.
A large and growing body of literature has investigated the prob-em by introducing low surface tension materials into the resintructure and coating formulations. These methods can be classi-ed into different categories including use of: (I) fully fluorinatedesins [15] or silicone resins [16–18] in coatings, (II) partially fluori-ated or siliconized resin in coatings through introducing fluorine19] or silicone monomers in different morphologies [20–26], silaneoupling agents such as vinyl alkylsilanes into the backbone ofcrylic or urethane binders [27,28] and grafting fluorine or sili-one groups to common organic polymer backbones [29,30], (III)dditives such as organically modified polysiloxanes and fluoro-urfactants to decrease surface tension of the coatings [31–33],nd eventually, (IV) physical blending of polymers [34–41]. Highxpense, environmental pollution and poor workability of fluorineompounds have led researchers to pay more attention to silicones.opolymers of silicone/acrylate (category II) have been synthesizedhrough various polymerization process; solution polymerization42–48], non-aqueous dispersion polymerization [49] and emul-ion polymerization [50–53]. Among the mentioned categories,hysical blending of silicone/acrylic resins has been recognized as
facile and flexible method to obtain weatherable coatings withuperior properties. It is more important for water-borne coatingsn which design of resins needs more sophisticated techniques [34].
Film formation, a challenging subject in emulsion water-bornelends coatings, has known to be important due to its impact onnal film properties. The main factors affecting this process containompatibility of the coatings’ components, glass transition tem-erature (Tg) of the resin, minimum film formation temperatureMFFT), particle size and particle size distribution of the blendedesins [54,55]. Poor compatibility between polysiloxane/acrylicesins in blends [38] has been overcome through various meth-ds such as grafting silanes onto the acrylic particles in ordero decrease interfacial tension between two different phases andross-linking of polymer particles in coalescence stage of film for-ation process [56]. Lepizzera et al. [39] have reported emulsion
lends of low Tg and high Tg emulsion polymer particles to achieve
nique properties in the films. They have investigated film forma-ion and mechanical properties of low/high Tg emulsion blends andeported an appropriate weight fraction of the high Tg emulsion
1.63–8.15 –185.55–205-55
polymer to obtain transparent and void-free films and also hightensile strength. Eckersley et al. [40] have studied the film forma-tion and mechanical properties of low/high Tg emulsion blendswith the aim of improving block resistance of the coatings. Fenget al. [41] have investigated the film formation of high (poly(methylmethacrylate) (PMMA)/low (n-butyl methacrylate-n-butyl acry-late) (BMA-BA copolymer)) Tg emulsion polymer blends and foundthat the PMMA particles retained their original spherical shapein the blends. Uniform dispersion of PMMA particles caused toproduce transparent films, while turbid films arise from cluster for-mation of PMMA particles. Chevalier et al. [57] evaluated the filmproperties of polystyrene/poly (n-butyl acrylate) emulsion blendsusing small-angle neutron scattering. Formation of clusters duringcoalescence stage of film formation at temperatures above the Tg
of polystyrene was prevented by employing core-shell morphologyof the same composition of the blends.
The influence of multimodality in latex particles on the filmformation process and therefore on final film properties has alsobeen investigated in literatures [58–61]. Creton et al. [62] studiedon deformation ability and uniform distribution of the particles invarious combinations of hard/soft particles with different sizes. Itwas observed that the critical volume fraction of small particlesrequired for obtaining uniformity in bimodal latex along with theleast void content. Also, poor packing of mono-size large particleslead to form voids in the film and thus weaken the film proper-ties. Geurts et al. [63] studied the effect of bimodality in particlesize distribution on MFFT of poly (BMA) latexes. They have foundthat the films with the same MFFT, but improved properties can beobtained using adequate particle size distributions.
In order to improve dirt pick-up resistance of water-borne coat-ings through enhancing hydrophobic character of the resin, twodifferent latexes were synthesized: BA-St acrylic via free-radicalemulsion polymerization and polydimethylsiloxane (PDMS) latexfrom a commercial monomer (i.e. Octamethyl cyclotetrasiloxanenamed D4) via cationic ring opening macro-emulsion polymer-ization. In this work, the parameters affecting polymerizationprocesses such as monomer concentration, type and amount ofmixed emulsifiers, initiator concentration, impeller stirrer speed
and temperature of reaction were optimized in such a way to pro-vide high stable emulsion blends with a compatible character andbimodal particle size distribution. The novelty of this work com-
J. Khanjani et al. / Progress in Organic Coatings 102 (2017) 151–166 153
F 290Dc imula
pbac
2
2
g(mtD(
ig. 1. Comparison between (a) commercial styrene acrylic copolymer (Acronal Tontamination with (b) dirt before rinsing, and (c) after rinsing with water as rain s
rises (a) successful preparation of stable blended latexes withimodal particle distribution from two totally different emulsionsnd (b) improving dirt pickup resistance without sacrificing otherharacteristics of the coating.
. Experimental
.1. Materials
Styrene (St) and butyl acrylate (BA) monomers in technicalrade were supplied by Simab Resin Mfg Co. (Iran) and acrylic acidAA) prepared from Sigma-Aldrich Chemical Company (USA). All
onomers were used without any purification. Octamethyl cycloetrasiloxane (D4) was supplied by SiSiB Silicones (South Korea).ifferent surface-active agents such as sodium dodecyl sulfate
), neat acrylic (AS-0) and an optimized blend (AS-S 72/28) latex films, followingtion.
cinate (DOSS) and sodium vinyl sulfonic acid salt solution (SVS)all from Sigma-Aldrich Chemical Company (USA) used as anionicemulsifiers and nonylphenol ethylene oxide-20 (Iconol NP-20 fromBASF Wyandotte corporation, Germany) as a nonionic emulsifierand ammonium persulfate (APS) from Merck (Germany) as aninitiator in the acrylic emulsion polymerization process. Dode-cylbenzene sulfonic acid (DBSA), employed simultaneously as anemulsifier and initiator in emulsion polymerization of D4, was gen-erously provided by Saveh Resin (Iran). Sodium bicarbonate fromMerck (Germany) was used as a buffer to adjust the pH of bothemulsions. Pigment black known as Printex-V was also supplied byEvonic degussa Ind. (Germany). Acronal T290D used as commer-
cial acrylic-styrene latex, as control sample for evaluationn of dirtresistance properties of prepared films was supplied by BASF Co.(Germany). De-ionized water was used to prepare all the solutionsand emulsions.
154 J. Khanjani et al. / Progress in Organic Coatings 102 (2017) 151–166
F ypes av
2
pedS9(ipamsfia8rm(pfiffi
ig. 2. Acrylic polymerization: Conversion as a function of time using different (a) tersus mixed emulsifier contents.
.2. Synthesis of acrylic copolymer latex
Semi-batch emulsion copolymerization was carried out throughre-emulsified monomer addition process in a 5 necked 1 L flaskquipped with a reflux condenser, stainless steel stirrer, samplingevice, N2 gas injection and feeding systems. Emulsifier type (SDS,DBS, DOSS, SVS and NP-20), emulsifier content (3, 4.5, 7.5 and
g), initiator content (0.30, 0.60, 1.20 and 1.50 g) agitation speed150, 200, 250 and 300 rpm) and temperature of reaction (70–90 ◦Cn 5 ◦C intervals) were considered as variables to optimize theolymerization process and yield high conversion. One factor at
time was applied to investigate the variables affecting the poly-erization process. A typical recipe for the preparation of a 50%
olid content product is given in Table 1. The reaction vessel wasrst charged with certain amounts of water, sodium bicarbonate,nd APS initiator. The solution was then stirred and heated up to0 ◦C. The polymerization was performed with a normal feedingate of 3.0 ml/min under N2 atmosphere. Then a pre-emulsion ofonomers (including BA, St, and AA) and mixed anionic/nonionic
3:1 weight ratio) emulsifiers were fed into the vessel. The rate ofre-emulsion feeding was tuned to 1/3 normal feeding rate at the
rst hour of the reaction and speeded up to normal feeding rate
or 2 h. The reaction was continued for about one hour at 90 ◦C. Atnal stage, the vessel was cooled down to room temperature and
ammonium hydroxide (98%) added to adjust the pH of the latex at9.
In order to measure the conversion of the polymerization dur-ing the reaction, samples were taken from the reaction media every30 min. The amounts of the samples were low so that the totalcomposition inside the reactor remains unchanged. First, 7 ppm ofhydroquinone was added to the samples in order to stop polymer-ization reaction, and then ethanol was used to coagulate the latexand let the sample to dry at 140 ◦C for 1 h.
2.3. Synthesis of PDMS latex
PDMS homopolymer was synthesized using D4 monomer inthe same reactor and same polymerization procedure as acryliccopolymers polymerization. DBSA content (1.63, 3.26, 4.89, 6.52and 8.15 g), type of emulsifier (DOSS, SDBS, SVS and NP-20), emul-sifier content (1, 2, 3 and 5 g), D4 monomer content (371.11, 381.11,401.11 and 411.11 g), temperature of reaction (70–90 ◦C in 5 ◦Cintervals) and agitation speed (110, 130, 150, 170 and 190 rpm)were varied to optimize the polymerization process and yield highconversion. A typical recipe of ingredients to obtain a 20% solid con-
tent PDMS is given in Table 2. The reactor was first charged withwater and DBSA and heated up to 85 ◦C and stirred at a constant rate.D4, Iconol NP-20, SDS, Sodium bicarbonate and water was mixedusing a magnetic stirrer for 15 min at room temperature and then
J. Khanjani et al. / Progress in Organic Coatings 102 (2017) 151–166 155
Table 3Components and variables in optimization of acrylic copolymer synthesis through macro-emulsion polymerization reaction.
Run Variables BA (g) St (g) AA (g) Water (g) APS (g) T (◦C) Emulsifier AgitationSpeed (rpm)
Stable: Latex was synthesized neither with coagula formation during polymerizatiNot stable: Latex was synthesized with a high amount of coagula and/or sediment
ed into the reactor. The polymerization was performed with a nor-al feeding rate of 3.0 ml/min under nitrogen atmosphere. Feeding
ate of pre-emulsion at the first hour of the reaction was tuned on.7 ml/min and thereafter on 1.6 ml/min. The reaction was contin-ed for 4 h at 85 ◦C. Thereafter, the reaction product was cooledown to room temperature.
.4. Preparation of the acrylic/PDMS blends
Optimized acrylic copolymer and PDMS polymer latexes werelended using a magnetic stirrer in different weight ratiosacrylic/PDMS blends in 0/100, 10/90, 20/80, 30/70, 40/60, 50/50,0/40, 70/30, 80/20, 90/10 and 100/0 wt.%). The blends were thenoated on glass substrates in 200 �m wet thickness and leave tory at room temperature. The films without any visual defects andppropriate drying time were considered to investigate in moreetails.
. Characterization
.1. Monomer conversion
In order to characterize both resins, monomer conversion (Eq.1)) was carried out [64].
(%) = (W1 × S + W2 − W3 − W4)/W0 × 100% (1)
here, W1 is the whole output of the latex, W2 is the amount ofel, W3 is the amount of initiator, W4 is the amount of emulsifiernd W0 is the amount of whole monomers.
.2. ATR-FTIR spectroscopy
ATR spectroscopy was used as a reliable and well recognized
ngerprinting method to identify the synthesized resins. ATR-FTIRpectrum was recorded using spectrophotometer Nicolet FTIR 460Czech Republic). Thin films (c.a. 10 �m DFT) of the latex were useds the test samples.
r particles sedimentation/floatation after polymerization.
3.3. Gel permeation chromatography (GPC)
Molecular weight and polydispersity index of synthesized poly-mers were determined using gel permeation chromatography(GPC), Waters 515-2410 model (USA), equipped with HT4-HT5columns and calibrated with standard polystyrene samples. Poly-mers dissolved in THF solvent and injected to the columns as thefluent phase. The elution velocity was 1 mm/min at 23 ◦C.
3.4. Particle size distribution measurement
Dynamic light scattering (DLS) technique using Malvern Zeta-sizer Nano ZS (UK) was applied to determine the size distributionprofile of the latex particles. The synthesized samples were dilutedwith water to 1 wt.% solutions and were analyzed at 25 ◦C.
3.5. Optical microscopy and EDX Si-mapping
In order to determine the adequate weight percentage of PDMSused in acrylic/PDMS blends with no defects, optical micrographswere prepared from the surface of acrylic, PDMS and differentblends of acrylic/PDMS films. Also, Surface and cross sectionalobservation was performed using SEM-EDX technique. Energy dis-persive X-ray analysis was used to evaluate the distribution ofsilicone component in the matrix of latex blends. Si-mapping fromthe surface and cross section of the sample films was carried outusing VEGA\\TESCAN Device made by Czech Republic. The samplefilms (1 × 1 cm) were placed in the vacuum chamber of the instru-ment, coated with a thin layer (15 nm) of Au through spattering,and then examined at various magnifications.
3.6. Contact angle and water up take measurements
Water contact angle and water uptake in coatings’ films were
considered as key factors and indication for dirt pickup resistance.Hydrophobic character of the films was assessed via static con-tact angle measurements. Static measurements were performedon a GBX-instrumentation digi-drop apparatus (Ireland) at 25 ◦C
156 J. Khanjani et al. / Progress in Organic Coatings 102 (2017) 151–166
Table 4Components and variables in optimization of PDMShomopolymer synthesis through macro-emulsion polymerization reaction.
Run Variables D4 (g) DBSA (g) Water (g) T (◦C) Emulsifier AgitationSpeed (rpm)
) Stable: Latex was synthesized neither with coagula formation during polymerizaa Oil Layer Top Seen.
sing the drop method. A 5 �l liquid drop was deposited onto theurface of the sample films with 250 �m thickness using a microm-ter syringe fitted with a stainless steel needle. The contact anglen both sides of the drop was measured to ensure the symmetry.lean and fresh water was chosen as test fluid to measure contactngles. The images were taken from each drop and subsequentlyhe contact angles were evaluated.
Water absorption (Aw) of acrylic and acrylic/PDMS films waseasured as follows: free films of different coatings were prepared
t first. Sample films in a determined weight (∼2 g) were placed intohe 20 ◦C water and weighted accurately after 14 days. The weightf dried films was also recorded and water uptake calculated usingq. (2):
w(%) = (W1 − W2)/W2 × 100 (2)
here, W1 is the film weight before drying and W2 after drying.
.7. Dirt pickup resistance
In order to quantify dirt pickup resistance of the latex films,rintex-V black pigment (0.2 wt.%), a pollutant model, was dis-ersed in water using SDS emulsifier (0.5 wt.%) via an acceleratedontamination-decontamination procedure. The method used wasifferent from ASTM D3719 standard because of difficulty for com-aring the results in different locations for exposure as well as longuration of the test [7]. As can be seen in Fig. 1a, the latex sam-le was applied on a glass plate (8 × 12 cm) with an applicator inet thickness of 200 �m and allowed to dry at room temperature
21 ± 2 ◦C and 50% relative humidity) for one day. The pollutantispersion (1.5 ml) was then casted onto a part of the film sur-
ace (4 × 6 cm) and placed in ambient temperature for about 30 minFig. 1b). The panel was rinsed with water as rain simulation andhen allowed to dry at room temperature (Fig. 1c). Color coordinatesere measured on both unpolluted and polluted part of each plate
or particles sedimentation/floatation after polymerization.
using Mini-Scan XE Plus portable color measurement instrument(USA). Dirt resistance was evaluated through lightness differences(�L) and color differences (�E) data. Acronal T290D, a commer-cial styrene-acrylic copolymer from BASF Co., was used as a controlsample.
3.8. Stress-strain analysis (SSA)
SSA was conducted to evaluate the mechanical characteristicsof the acrylic coating blended with different weight percents ofthe synthesized PDMS latex. In this experiment, the liquid poly-mers of the acrylic and acrylic/PDMS latexes were casted into thepolytetrafluoroethylene molds and leave to dry at ambient tem-perature. The films were removed from the molds and cut intospecimens in 13 cm long and 2.5 cm wide. Free films as test speci-mens were conditioned at 23 ± 2 ◦C and 50 ± 5% relative humidityfor 24 h according to the ASTM D 2370 and subjected to tensile load-ings and extensions using M500-50 AT Tensile Tester with a 2 knload cell was from Testometric Company (UK). The testing machinewas equipped with a device for recording the tensile load and theamount of separation of the grips. The rate of separation of thegrips was adjusted to 50 mm/min. The load versus extension wasrecorded for each sample. A minimum of five specimens for eachsample was tested. Young’s modulus, tensile strength, stress andelongation at break were calculated from the obtained data.
4. Results and discussion
4.1. Optimization of emulsion polymerization of acryliccopolymer
Interfacial phenomena play an important role in the perfor-mance of the heterogeneous reaction systems such as emulsionpolymerization. Type of the emulsifiers, content of the emulsifiers
J. Khanjani et al. / Progress in Organic Coatings 102 (2017) 151–166 157
F ts, (ba
anfcpmtavs
tts
iTpca
aartmtatvfi2
ig. 3. D4 polymerization: (a) conversion vs. reaction time in different DBSA contennd (d) amounts of mixed emulsifiers.
nd initiator concentration as factors affecting the interfacial phe-omena play not only an important role in monomer conversion in
ree radical polymerization process, but also on stability of the latexolloidal particles [65]. Also circumstances of the polymerizationrocess such as agitation speed and polymerization temperatureay affect both conversion of the reaction and colloidal stability of
he latex. Therefore, several experiments in 19 runs were designednd performed according to Table 3 in order to optimize the con-ersion of the polymerization process and final properties of theynthesized acrylic latex.
Emulsifiers are generally required to stabilize the colloidal sys-em of the latexes and prevent coagulation and adhesion of particleso the reactor wall and agitator and also provide stability during theervice life of the latex.
Effect of different mixed emulsifiers on the yield and stabil-ty of the polymerization was investigated (AS1 to AS4 runs inable 3) and as it can be clearly seen from Fig. 2a, highest rate of theolymerization was observed in the presence of mixed emulsifiersonsisting of anionic emulsifier SDS and non-ionic one, NP-20, and
stable product with highest conversion of 96% was obtained.Contents of the optimum mixed emulsifiers were varied in the
mounts of 3.0, 4.5, 6.0, 7.5 and 9 g (equals to 1.0, 1.5, 2.0, 2.5nd 3.0 wt.% of the monomers, respectively) in AS1 and AS5–AS8uns. Based on the related final conversion data stated in Table 3,he emulsifier content must not be less than 2.0 wt.% of total
onomers. Rate of the polymerization and final conversion illus-rated in Fig. 2b, confirms this claim. It is worth mentioning that themount of the emulsifier should be kept at optimum level in ordero avoid coagula formation during polymerization. Coagula content
ersus mixed emulsifier content was depicted in Fig. 2c. From thisgure it can be concluded that in the emulsifier content lower than
wt.% polymerization was considerably unstable and a high level of
) final conversion vs. DBSA contents, and conversion vs. time for different (c) types
the coagulated particles were formed in the reaction media. Usingminimum practical level of the emulsifiers that provide stable col-loidal latex would be also desirable according to the properties ofthe coatings prepared by the latex. Therefore, 2.0 wt.% of anionicemulsifier SDS and non-ionic one, NP-20, can be considered as anoptimum amount for further runs.
Initiator content, the third parameter, has been varied from0.29 g (1.27 × 10−3 mol L−1) to 1.5 g (10.9 × 10−3 mol L−1) in poly-merization process. Data reported in Table 3, shows that the highestconversion of 97% was obtained using 1.18 g (8.6 × 10−3 mol L−1)in AS11 run. In Fig. 2d, the changes in conversion versus timeduring polymerization have been shown. The polymerization rateincreases with increasing initiator concentration as previouslyreported in the literature [25]. The initiator should be used inoptimum level; in high concentration, it leads to decrease in themolecular weight of the synthesized polymer that is not desirablefor the coatings and on the other hands, in low concentration finalconversion of the polymerization decreases.
Agitation speed was considered as a processing factor affectingthe conversion of the polymerization reaction. Excessive agitationwould produce greater mechanical forces or hydrodynamic shearstresses. Shear forces used to provide the heterogeneous emulsionpolymerization with efficient mixing and heat transfer also haveinfluence on the colloidal stability of latex particles during poly-merization [65]. AS13–AS16 runs was designed with the speeds in150, 200, 250 and 300 rpm. The related conversion data reported inTable 3 showed that agitating in 150 rpm (lowest impeller speed)yielded the highest conversion (98%). This high conversion can beattributed to low shear force caused by the lowest mixing speed
of 150 rpm in comparison with the other agitation speeds. Stirringsignificantly affects the number of produced polymer particles andthe polymerization rate per particle. Final conversion was increased
158 J. Khanjani et al. / Progress in Organic Coatings 102 (2017) 151–166
acryl
dp
rt(impEtAP
4
owmceT
varsDi0Hnbtieiti
Fig. 4. FTIR spectra of (a) PDMS polymer and (b)
ue to lower collision in polymer particles and the probability ofarticles’ coagulation caused by low agitation speed [66].
As can be seen from Table 3 (AS16 to AS19 runs), there was aise in final conversion of polymerization as the reaction tempera-ure increased from 70 ◦C to 90 ◦C. The maximum final conversion99.5%) was reached for the reaction carried out at 90◦ C. Increas-ng temperature promotes the kinetic energy of the monomer
olecules and improves monomer transfer into the micelles andolymer chains and increases the rate of the polymerization [67].ventually, based on the conversion data and colloidal stability ofhe latex particles in all the runs, the sample obtained from runS19 was selected as optimum acrylic latex to be blended with theDMS latex.
.2. Optimization of D4 emulsion polymerization
Optimization of D4 conventional polymerization in order tobtain PDMS polymers with high conversion and stable latexesere carried out in different levels of the factors affecting poly-erization process containing DBSA content, emulsifiers type and
ontent, D4 concentration, temperature and agitation speed. Thexperiments were designed in 24 runs and performed according toable 4.
The amounts of DBSA as cationic initiator and emulsifier, wasaried in the range of 1.63 g (0.01 mol L−1) to 8.15 g (0.05 mol L−1),s stated in Table 4 (SL1–SL5 runs). In Fig. 3a conversion of theeaction was plotted against polymerization time. As it can beeen from this figure, in the graphs of 0.01, 0.02, and 0.03 mol L−1
BSA, the rate of polymerization in different times are approx-mately constant, and also increasing the amount of DBSA from.01 to 0.03 mol L−1 rise to higher conversion at a certain time.owever, adding DBSA in the amounts of 0.04 and 0.05 mol L−1
ot only, changes the rate of polymerization through the reaction,ut also cause to decrease in the conversions over the reactionimes. This can be explained by the fact that DBSA acts as bothnitiator and emulsifier in polymerization of D4 [68]. DBSA in an
xcess amounts, provides many active sites in polymerization andncreases the reaction sites, although can retard D4 diffusion intohe polymer droplets due steric hindrance effect of DBSA moleculesn the interface of the aqueous/organic phase.
ic-styrene copolymer extracted from AS19 latex.
Comparison of final conversions as a function of DBSA concen-tration is figured out in Fig. 3b. It is obvious that 0.03 mol L−1 DBSAlead to the maximum conversion in SL1–SL5 runs and thus wasconsidered constant for the next runs.
Adequate colloidal stability in emulsion polymerization sys-tems can be achieved by using a blend of the anionic/nonionicsurfactants, because of the hybrid effects of the steric and electro-static interactions [37,39,66]. As can be clearly seen in Fig. 3c, withan investigation on the effect of the mixture emulsifier types onpolymerization conversion, it can be concluded that in the pres-ence of the mixture emulsifiers consisting of anionic emulsifierSDS and nonionic emulsifier NP-20 with the ratio of 3:1 w/w, D4polymerization was progressed with higher yield of conversion incomparison with other mixed emulsifiers (SL3 and SL6–SL8 runs)and considered constant for the next runs mentioned in Table 4.
In order to investigate the effect of mixed emulsifier concen-tration on conversion of polymerization reaction of D4, differentconcentrations relative to weight of monomers (i.e. 1, 2, 3, 4 and5%) were selected in SL9–SL11 runs. The final conversions are statedin Table 4 and the trends of polymerization rates are revealed inFig. 3d. The results showed that increase in SDS/NP-20 emulsi-fier’s content leads to increase in final conversion from 64% (for 1%emulsifier) to maximum 89% (for 4% emulsifier). Conversion yieldof polymerization was high upto the emulsifier concentration of2% while there cannot be seen high conversions at higher concen-trations of emulsifiers up to 5% emulsifier. In spite of the fact thatincrease in emulsifier content can provide more stable polymeriza-tion, using emulsifier has some limitations due to the performanceproperties of the coatings during the service life of the coating.Therefore, the lowest possible concentration of SDS/NP-20 mixedemulsifiers (2 wt% in SL10 run, Table 4) in which successful poly-merization was processed with high conversion yield was selectedas the optimized level of emulsifier in these runs. Based on the the-ory of the emulsion polymerization [65], it has been known thatthe size of the latex particles continuously grows during the poly-merization. Thus, more emulsifier molecules are needed in orderthat the polymerization remains stable. According to our experi-
ence, the less content of the emulsifier resulted in less number ofparticles with larger sizes. In this state, bluish shining cannot bepractically seen. When polymerization progressed, more emulsifier
rganic Coatings 102 (2017) 151–166 159
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olecules cannot be adsorbed on the surface of the latex parti-les. As a result, the ratio of the protected surfaces by emulsifierolecules decreases causing unstable polymerization or even sep-
ration of the monomer droplets from micelles. But, if the amountf emulsifiers used is more than what needed, number of the latexarticles is increased and particle size highly is decreased; it meanshat surface area is considerably enlarged. Thus, intermoleculareaction is so important and quickly increased viscosity causinghe dispersion and continuing of the polymerization reaction to beifficult [67,68].
Regarding the effect of D4 monomer concentration (80, 90, 100,10 and 120 g) on polymerization SL3 and SL13–AS16 runs wereesigned. Final conversion and colloidal stability reported in Table 4or these runs showed that consumption of 100 g (20 wt.% totaleight of formulation) D4 (SL3 run) provided the highest conver-
ion along with the stable latex. In higher concentrations in SL15nd SL16 runs, an oil layer of non-reacted monomers was observedn top of the latex.
In the temperature series of experiments (Table 4, SL17–SL20uns), the polymerization was performed in the range of 70–90 ◦Cith 5 ◦C intervals. As expected and described in Section 4.1, with
ncrease in the temperature, the final conversion was increased upo 91% in SL20 run. The effect of agitation rate, as another process-ng factor impressing the polymerization rate and conversion wasvaluated under different stirring speeds (110–190 rpm in 20 inter-als). The highest and the lowest conversion of the polymerization94 and 86%) were obtained in the speeds of 110 rpm and 190 rpm,espectively. According to the Arai theory [69], it can be explainedy considering monomer mass transfer from monomer dropletso the polymer particles formed in the aqueous phase. Finally, theample obtained from run SL21 was selected as optimum PDMSatex due to having high conversion and colloidal stability to blend
ith the optimized acrylic latex.
.3. FTIR analysis
The molecular structure of the synthesized PDMS and acrylic-tyrene was confirmed by the FTIR spectra, provided in Fig. 4. PDMSpectrum in Fig. 4a, showed strong Si O Si stretching absorp-ions at about 1071 and 1023 cm−1, which are characteristic of asymmetric and symmetric stretching of Si O Si in siloxane back-one. In addition, C H (in Si CH3) in bending and rocking peaksere observed at 1260 cm−1, and 802 cm−1. The peak observed at
965 cm−1 can be assigned to stretching mode of C H in CH3 [56].The FTIR spectra of the synthesized acrylic-styrene excluded
rom AS19 latex was provided in Fig. 4b. The appearance of theand at 3070 cm−1 can be attributed to the stretching movementsf the C H band of the phenyl group. Also, absence of the stretchingand relative to the double band at 1630 cm−1 illustrates com-letion of the polymerization reaction. Additionally, the observedand at 1733 cm−1 is related to the carbonyl group and a smalleak at 1600 cm−1 is an evidence for double bond’s presence in thehenyl group confirming successful synthesis and formation of thecrylic-styrene copolymer [24].
.4. Particle size distribution and molecular weight
In emulsion polymerization, the size distribution of the latexarticles and the molecular weight of the polymer are two key prop-rties which would affect the coatings mechanical performance70]. For this reason, particle size distribution of the PDMS andcrylic Latexes, and a typical blend of acrylic/PDMS (1:1 weight
atio) were measured and also, the molecular weight of the poly-ers was determined.Analysis of the particle size for each optimized PDMS and acrylic
atexes, were shown in Fig. 5a. As it can be observed a mono modal
Fig. 5. Particle size distribution for (a) PDMS and the acrylic-styrene lattice individ-ually, and (b) bimodal distribution of PDMS/acrylic blend latex (1:1 weight ratio).
distribution spread from 25 nm to 250 nm with an average particlesize of 88 nm for PDMS polymer, and 75 nm to 400 nm with an aver-age particle size of 155 nm for the acrylic copolymer were resulted.In Fig. 5b, two particle size distribution graphs were indicated; oneof them was calculated from the size distribution data obtainedfrom individual acrylic and PDMS polymers with the assumptionof blending in 1:1 weight ratio, and the other one shows the distri-bution produced by the actual blend of acrylic/PDMS in 1:1 weightratio after storage at room temperature for one month. The specificgoal of analysis on the actual blend was to experimentally deter-mine the stability of the blend latex during the storage. As it is clear,the particle size distribution of the actual blend is completely inagreement with the anticipated one. This shows the compatibil-ity and colloidal stability of the mixed particles in the blend. Inthe meantime, blends of acrylic as a hard film forming polymerand PDMS as a soft non-film forming polymer particles in differ-ent sizes caused to a bimodal distribution. Latex dispersions withcontrolled particle size distributions have been advised so as toimprove the film formation. Addition of smaller particles leads toimproved packing of the particle which in turns causes to reducethe void content in the bulk of the film [71].
One purpose of determining molecular weight of the synthe-sized polymers is characterization and the second one is to provide
better predictions for mechanical properties of the coatings, sothat a more accurate and applicable model may be obtained.GPC chromatographs of optimized PDMS and acrylic Latexes areshown in Fig. 6. PDMS latex had average molecular weight of
160 J. Khanjani et al. / Progress in Organic
Fs
MaMd
4
sctilVears9
tIaplfipwaaoc
oPpf
pmAsmo
ig. 6. GPC chromatographs of optimized latex of: (a) PDMS polymer, and (b) acrylic-tyrene copolymer.
n = 129,000 g mol−1 (and Mw = 290,000 g mol−1) and PDI of 2.23,nd the optimized acrylic latex had average molecular weight of
n = 61,000 g/mol (and Mw = 304,000 g mol−1) and PDI of 4.93 asetermined by GPC.
.5. Characterization of acrylic/silicone blends
A composite of an acrylic and a silicone has the potential to pos-ess the good film forming and mechanical properties of the acrylicoupled with low surface energy, water resistance, and resistanceo outdoor weathering of silicones, with each component overcom-ng the weakness of the other. Blending PDMS emulsion with acrylicatex would be a facile method to produce silicone-modified acrylic.arious blends of optimized PDMS/acrylic latexes and surface prop-rties of thereof films are presented in Table 5. AS/S stands forcrylic/PDMS blends and the number in the coding samples rep-esents the percentage of PDMS in the blend. For instance, AS/S1tands for an acrylic/PDMS blend containing 10 wt.% of PDMS and0 wt.% of the acrylic.
The appearance of the films was visually evaluated in Fig. 7 andhe defects such as cracks and craters were recorded in the table.ndeed, the films of the optimized acrylic emulsion formed withoutny defects at room temperature (i.e. above Tg of the polymers andolymer blends). Incorporating PDMS up to 30 wt.% into the acrylic
ed to clear films with no defects. Transparency in AS-S1 and AS-S2lms can be explained by appropriate film formation of the acrylicolymer particles as a dominate phase and adequate coalescingith PDMS particles during film formation. Semi opaque appear-
nce in AS-S3 film would be a clue for starting the phase separationnd/or disruption of uniform particle packing at the second stagesf film formation [72]. Thus, the highest silicone amounts whichould be incorporated into the acrylic was about 30 wt.%.
PDMS disrupts the uniform particle packing at the second stagesf film formation required for transparent films. The blends withDMS ratios more than 30 wt.% all were turbid, because of the inap-ropriate coalescence of small PDMS particles and consequent skinormation [72].
Short drying time in coatings is an advantage from applicationoint of view. As it was reported in Table 5, drying time for film for-ation was increased gradually with increase in PDMS latex ratios.
lso, in addition to suitable film formation of AS-S1 and AS-S2amples, the low drying times (21 and 22 min) of these silicone-odified latexes can be considered as an advantage of blending
ptimized PDMS with acrylic emulsion. The longer drying time can
Coatings 102 (2017) 151–166
be assigned to skin formation [72] caused by coalescing of sili-cone droplets near the surface of the films, and retarding waterevaporation at the first stage of film formation.
4.6. EDX analysis
PDMS particles need to be uniformly distributed in acrylicmatrix to provide acceptable properties in the coating. Energy-Dispersive X-ray Si-mapping has been used to observe thedistribution uniformity of PDMS moieties in acrylic blend films andanalyze the PDMS composition in the bulk and on the surface of var-ious optimized blend films. Fig. 8 depicts EDX Si-mapping imagesprepared from the surface and cross-section of the optimized latexfilms (acrylic/PDMS: 90/10 and 80/20 w/w). As it is clear from thefigure, PDMS particles are well distributed on the surface and alsoin the bulk of the film. This result comes from the optimum usage ofPDMS and bimodal distribution of latex particles in the optimizedblends.
Si compositions on the surface and bulk of the optimized filmsobtained from EDX analysis are reported in Table 6. According tothe results, it can be stated that Si content as an representative foramounts of PDMS are higher in the bulk than the surface of the filmsfor both blends. This in turns shows the appropriate distribution ofthe PDMS particles in the bulk of the optimized blends’ films.
4.7. Dirt pick-up properties
Different kinds of dirt can be adsorbed on the surface of thecommon exterior coatings causing dirt pickup. Dust in the form ofdissolved or dispersed in water absorb on the surface of the coat-ings. Then, the films absorb water, leading dust into the surfacecapillaries, in result of permanent contamination of the coatings.As a result, increase in water contact angle and decrease in wateruptake would improve dirt pick-up of the films.
4.7.1. Contact angle and water uptake measurementsA series of acrylic/PDMS blend latexes containing PDMS in the
range of 10–28 wt.% were prepared. Two key factors, static watercontact angle (WCA) and water uptake, of the films were measuredand the results were summarized in Table 7. Also, typical images ofwater droplets on the surface of the films are illustrated in Fig. 9.Incorporating PDMS latex into acrylic emulsion, in the amounts asmentioned in Table 7, gave rise to increase in water contact angleson the surface of acrylic/PDMS latex films from 66◦ to 96◦. Smallcontact angles (<90◦) correspond to high wettability, while largecontact angles (≥90◦) correspond to low wettability. Dirt adhesion,dirt adsorption, and dirt absorption are all affected from surfacewettability [73]. Thus, addition of optimized PDMS latex to theacrylic latex can improve dirt pickup resistance of the coating.
Water uptake data reported in Table 7 indicates that PDMSingredient in the blend films decreases water uptake in acryliccoatings up to 44.8%, which is a noticeable value. Water uptakealteration over 7 days is indicated in Fig. 10. It can be seen in thisfigure, the water uptake in each day decreases drastically with addi-tion of PDMS latex to the blend films. Permeated water in the acrylicfilms dissolves the emulsifiers and initiators. Migration of dissolvedingredients from bulk to the surface of the film creates capillarytubes in the film which in turns caused an increase in water uptake[74]. In blend films, PDMS particles act as a barrier because of betterpacking with acrylic particles and also inherent hydrophobic char-acter, so they retard the permeation of water into the film. Bimodalparticle size distribution and short drying film of the optimized
bend films confirm the claims, too.
Furthermore, water uptake in acrylic films shows an increasingtrend over all seven days however, in all blend films it grows upto 4 days and after that it becomes constant. Various coatings have
J. Khanjani et al. / Progress in Organic Coatings 102 (2017) 151–166 161
s, app
dct
4
mei
Fig. 7. Photos of dry acrylic (AS-0), and acrylic/PDMS blend film
ifferent saturation limits for water uptake which is affected byhemistry and morphology of the coating. Addition of PDMS latexo acrylic emulsions changes this value to shorter times.
.7.2. Color measurements
Dirt pickup resistance was also investigated by color measure-
ents performed on samples before and after contamination withqual amounts of an aqueous dispersion of Printex-V as illustratedn Fig. 1c. Acronal T290D, a commercial styrene acrylic copolymer
lied in wet film thickness 200 �m. See Table 5 for sample codes.
and control sample, AS-0 as a neat acrylic, and AS-S 72/28 as anoptimized blend were used in this evaluation. Color coordinatesdata obtained from color measurements based on CIE 1976 L*a*b*system and color difference formulas (45◦/0◦ geometry) are sum-marized in Table 8. The word “Contaminated” in the table stands
for the samples after rinsing with water and drying at ambienttemperature. Taking into consideration transparency of the filmsand the effects of the substrate on color coordinates data, colormeasurements were performed on the plates located on a white
162 J. Khanjani et al. / Progress in Organic Coatings 102 (2017) 151–166
Fig. 8. EDX Si mapping on the: (a) surface and (b) of the cross-sectional area of acrylic/PDMS: 90/10 w/w, and (c) surface and (b) surface and (d) cross-sectional area ofacrylic/PDMS: 80/20 w/w.
Table 5Various blends of optimized PDMS/acrylic latexes and surface properties of thereof films (200 �m, WFT).
Sample No. AS/Si latexes ratios Film Appearance (25 ◦C) Film defects Drying Time (min)
AS-0 100/0 (100% Acrylic) Clear-No tack No defects 23AS-S1 90/10 Clear -No tack No defects 22AS-S2 80/20 Clear -No tack Crater formed 21AS-S3 70/30 Semi opaque-No tack Crater decreased 25AS-S4 60/40 turbid-No tack Crater decreased 26AS-S5 50/50 turbid -No tack Crater developed 27AS-S6 40/60 turbid -No tack Continued crater development 30AS-S7 30/70 turbid -No tack Continued crater development 33AS-S8 20/80 turbid -No tack Continued crater development 47AS-S9 Oct-90 turbid -No tack Big crater through the film 56AS-S10 0/100 (100% PDMS) Clear and tacky Big crater through the film 47
Table 6Si EDX analysis of silicone component on the surface and cross-section of optimize acrylic/PDMS blends’ films.
Sample Si on the surface (%) Si on the cross section (%) Ratio of Si content on the surface to the cross section
L* = Lightness, a* = bluish-green/red-purple hue component, and b* = yellow/blue hue com
Fc
shpbi
ings toughness, elevating from 108 MPa in AS-0 pure acrylic film to
ig. 10. Variation of water uptake for latex hybrid films versus PDMS latex (Si)ontent. AS stands for acrylic-styrene copolymer latex.
urface. L*, a* and b* stands for lightness, bluish-green/red-purpleue component, and yellow/blue hue component, respectively. Dirt
ickup was evaluated through L* and lightness differences (�L)etween contaminated and uncontaminated part of the films. As
t is clear, �L values calculated for AS-S 72/28 (0.6) compared to
3.34
ponent.
Acronal T290D (4.05) and AS-0 samples (3.35), proved significantimprovement in dirt pickup resistance in acrylic/PDMS blend film.Color differences (�E) values confirmed the mentioned result aswell.
As an overall result, optimized acrylic/PDMS latex coatingsadsorb much less dirts and contaminant in comparison withunmodified acrylic films.
4.8. Mechanical properties of the films
Mechanical properties such as tensile strength, elongation, andtoughness are important characteristics of outdoor coatings [75].That is, these coatings usually are subjected to tensile and compres-sive forces occur in actual weathering conditions and such forcescan cause expansion and compression in the coating during its ser-vice life. Mechanical properties of water borne coatings depend oncomposition and film formation properties of the coating.
For comparison, the mechanical property of pure acrylic andoptimized blends of PDMS/acrylic films prepared using were mea-sured. Fig. 11 shows the tensile stress–strain curves of those filmsand the mechanical parameters are listed in Table 9. It can be con-cluded from the figure that incorporation of PDMS into the acrylicemulsion enhances the stress required to produce a certain strainin films. Test results show Young’s modulus as an index of the coat-
279 MPa in AS-S72-28 film. Toughness can be defined as the abilityof a coating to withstand an impact without cracking or breaking[76] and dependents on the nature of the polymer or polymers used
164 J. Khanjani et al. / Progress in Organic Coatings 102 (2017) 151–166
Table 9Mechanical properties of the films prepared from the acrylic/PDMS latex blends using stress-strain analysis.
Sample PDMScontent in Latexblends (w/w%)
Young’s modulus (MPa) Tensile Strength (MPa) Elongation at break (%) Work of Break (J)
[1] J.V. Koleske, Paint and Coating Testing Manual, Fourteenth Edition of the
ig. 11. Tensile stress-strain curve of the acrylic/PDMS blend films in the optimizedone.
n the coating. Besides, tensile strength and elongation at breakpsurges from 1.8 MPa and 120% in AS-0 acrylic film to 7 MPa and90% in AS-S72-28 modified acrylic film, respectively. Coincident
ncrease in data for elastic modulus and elongation at break in thecrylic/PDMS latex blends comes back to elastomeric character ofDMS as a well-known elastomeric polymer. Increased elongationt break is originated from the PDMS part as a soft segment, andmproved elastic modulus has been induced by the acrylic part as
hard segment in the composition. As a matter of fact, the goal ofhe blending was generally to have advantages of both individualystems in a particular blend.
Since the blends had higher tensile strength and elongation atreak compared to the acrylic latex, it is expectable that work ofreak values that can be calculated from the surface area below thetress-strain curves, would increase. In other words, more energys needed to break a blend sample compared to the acrylic one. Inpite of relative poor mechanical strength of silicone resins in com-arison to acrylic resins, the SSA analysis revealed that the obtainedilicone/acrylic emulsion blends having bimodal particle size distri-ution, in appropriate ratios can lead to 158% increase in modulus,550% increase in tensile strength and 475% increase in elongationt break. Also, work of break increased as much as 1200%. In otherords, optimal weights of PDMS latex improve both strength andexibility of the acrylic films. This can be described by uniform dis-ribution, proper compatibility and packing of PDMS and acrylicatex particles, inter diffusion of the polymer molecules across the
rea of contact between the particles, and thus appropriate film for-ation in the film which leads to improve mechanical properties
f acrylic exterior coatings.
.8 ± 0.2 590 ± 11 3447 ± 28
.4 ± 0.3 602 ± 5 4158 ± 19
.0 ± 0.1 690 ± 17 4830 ± 39
5. Conclusion
With the goal of reducing dirt pickup in water based coatingswithout sacrificing the characteristics of the coating, stable styrene-acrylic and PDMS latexes were synthesized to prepare latex blendswith bimodal particle distribution.
Styrene-acrylic copolymer latex as a common polymer for usein waterborne coatings was synthesized through pre-emulsifiedmonomer addition process. Type and content of mixed emul-sifiers, initiator content, agitation speed, and temperature ofthe reaction were optimized in order to yield colloidal stableacrylic latex with high conversion. An optimized sample (AS19in Table 3) was selected as optimum acrylic latex to be blendedwith the PDMS latex. Latex of PDMS homopolymer was synthe-sized via cationic ring opening macro-emulsion polymerization ofoctamethyl cyclotetrasiloxane (D4) and optimized by varying theparameters such as DBSA content as both initiator and emulsifier,content of mixed emulsifiers, D4 monomer content, temperature ofreaction, and agitation speed. SL21 sample in Table 4 was selectedas optimum PDMS latex due to having high conversion and colloidalstability to be blended with AS19 acrylic sample.
Optimized samples of both latexes were blended in differentweight ratios. Appropriate weight ratios in PDMS/acrylic blendswere determined with the aim of lowering dirt pick up in waterborne acrylic coatings without sacrificing mechanical and phys-ical properties of the coating. Optical microscopy, EDX analysis,static water contact angle and water uptake measurements, andSSA analysis were used for characterization of PDMS/acrylic latexblends. The highest valid PDMS content in blends was about 30 wt.%PDMS in acrylic latex. Furthermore, it was found that the amount ofPDMS latex, uniform and appropriate distribution of PDMS particleson the surface and cross-sectional area of blend films, and particlesize of PDMS latex (in a manner to provide bimodal particle sizedistribution in blends), were the main factors affecting film forma-tion and thus the performance of acrylic latex with even improvein mechanical properties. Increase in contact angles from 66◦ foracrylic to 96◦ and 44.8% decrease in water uptake in acrylic/PDMSfilm (72/28; w/w) were observed for hydrophobic character oflow adsorbent modified acrylic, which is a critical property in lowdirt pick up coatings. Color and specially lightness measurementsperformed on contaminated films, confirmed contact angle andwater uptake results. Eventually, it was concluded that optimizedacrylic/PDMS latex coatings adsorbed much less dirts and contam-inant in comparison with unmodified acrylic films.
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